1. Field of the Invention
The present invention relates generally to systems and methods for sculpting materials, and more particularly to a laser ablation system and method for sculpting a lens in a cornea.
Lasers have been used for several years to sculpt materials into very precise shapes. Excimer lasers are now widely used to ablate tissue in a variety of surgical procedures, particularly for corneal ablation during refractive surgery. The exposure of the tissue is typically controlled to produce a desired change in corneal shape. The change in corneal shape may be intended to correct a refractive error of the eye so as to eliminate the need for corrective eyeglasses, or may be intended to remove a pathology from the eye.
Known laser eye procedures generally employ an ultraviolet or infrared laser to remove a microscopic layer of stromal tissue from the cornea of the eye to alter its refractive power. The laser removes a selected portion of the corneal tissue, often to correct refractive errors of the eye. Laser ablation results in photodecomposition of the corneal tissue, but generally does not cause significant thermal damage to adjacent and underlying tissues of the eye. The irradiated molecules are broken into smaller volatile fragments photochemically, directly breaking the intermolecular bonds.
Selective photoablation of corneal tissues benefits from precise control over a laser beam. Control over the distribution of the ablative laser energy across the cornea may be provided by a variety of systems and methods, including the use of ablatable masks, moveable apertures, scanning systems that move laser beams of varying cross-section across the cornea, and the like. These laser control systems generally vary the profile of the laser beam, and thus the ablation area on which the laser impinges on the eye. As the ablation depth generally varies with the amount of laser energy, the distribution of laser energy across the laser beam is often kept as uniform as possible. The goal of this uniform energy distribution is to remove the corneal tissues uniformly throughout the laser cross-section. As excimer lasers produce laser beams as a series of laser pulses, the total ablation is often calculated as a series of ablations of uniform depth.
For laser refractive surgery to have an optimal result, the sculpting process should accurately remove corneal tissues so as to change the refractive characteristics of the eye in the desired manner. The tissues targeted for removal will generally be lens-shaped, and this lens-shaped ablation should often be surrounded by a smoothly tapering transition zone. Such a total ablation can only be approximated by the series of pulse ablations produced with most pulsed excimer lasers. This can result in ablations having undesirably abrupt changes in depth and/or staggered edges.
Several techniques have been proposed to smooth ablations. One proposal is to smooth the sharp edge of an ablation formed from an imaged aperture by defocusing the laser beam. An alternate proposal is to move the laser beam across the corneal surface between pulses so that the sequential pulses only partially overlap. Although refractive laser surgery using such approaches might be effective, the final ablations can often be less smooth than is desired. Known methods for defocusing of the laser beam may also reduce the accuracy of the overall refractive correction. Although partially overlapping sequential laser pulses can prevent the ablation edges of separate pulses from lining up, the size of each pulse edge is unaffected. Additionally, work in connection with the present invention has found that the precise shape of the actual ablation produced by a uniform laser pulse generally differs somewhat from the uniform ablation depth that has been theoretically predicted. Hence, the total ablation region can differ significantly from even the approximate lens shape that is intended.
In light of the above, it would be desirable to provide improved laser systems and methods for sculpting with lasers. It would be particularly desirable to provide new techniques for smoothing the ablations produced by lasers, especially the corneal ablations of laser refractive surgery. It would further be desirable if these improved techniques minimized unintended variations in the ablation depth, and did not significantly add to the cost or complexity of the laser systems.
2. Description of the Background Art
The following references are herein incorporated by reference in their entirety: U.S. Pat. No. 5,646,791 for xe2x80x9cMETHOD AND APPARATUS FOR TEMPORAL AND SPATIAL BEAM INTEGRATION;xe2x80x9d U.S. Pat. No. 5,683,379 for xe2x80x9cAPPARATUS FOR MODIFYING THE SURFACE OF THE EYE THROUGH LARGE BEAM LASER POLISHING AND METHOD OF CONTROLLING THE APPARATUS;xe2x80x9d U.S. Pat. No. 5,610,733 for xe2x80x9cBEAM-HOMOGENIZER;xe2x80x9d U.S. Pat. No. 4,547,037 for xe2x80x9cHOLOGRAPHIC METHOD FOR PRODUCING DESIRED WAVEFRONT TRANSFORMATIONS;xe2x80x9d U.S. Pat. No. 5,685,998 for xe2x80x9cMETHOD OF MINIMIZING DIFFRACTION GROOVE FORMATION ON LASER ETCHED SURFACES;xe2x80x9d and U.S. patent application Ser. No. 08/968,380, for xe2x80x9cMETHOD AND SYSTEM FOR LASER TREATMENT OF REFRACTIVE ERRORS USING OFFSET IMAGING,xe2x80x9d as filed Nov. 12, 1998.
The publication xe2x80x9cDIFFRACTIVE SMOOTHING OF EXCIMER LASER ABLATION USING A DEFOCUSED BEAMxe2x80x9d by McDonnel et al., published in Refractive and Corneal Surgery, Volume 10 (January/February 1994) describes a technique for smoothing ablations and is herein incorporated by reference in its entirety. An article entitled xe2x80x9cAXIAL AND TRANSVERSE DISPLACEMENT TOLERANCES DURING EXCIMER LASER SURGERY FOR MYOPIAxe2x80x9d by Shimmick et al., SPIE Ophthalmic Technologies, Volume 1423, page 140 (1991) may be relevant, and is also incorporated herein by reference.
The present invention generally provides improved systems and methods for sculpting a material to effect a predetermined change in shape by ablating a region of the material. The techniques of the present invention generally improve the smoothness and accuracy of the ablated shape by directing a plurality of laser beams toward the targeted region so that the beams strike differing areas of the material. The beams will generally be produced by separating a single laser beam, the single beam comprising a series of laser pulses. The ablation areas may partially overlap during at least some portion of the ablation procedure, and/or they may be separated during at least some portion of the procedure. Regardless, the edge depth for each pulse at these areas can be significantly less than that of a pulse directed at a single ablation area, as might be produced by the unseparated beam. The methods and systems of the present invention manipulate the multiple beams so as to alter the ablation pattern formed from the differing beam areas, thereby allowing more accurate removal of a smooth region from the material, particularly when removing a lens-shaped region of corneal tissue using a pulsed laser.
The present invention also makes use of blurred ablation edges. Surprisingly, work in connection with the present invention has found that these blurred edges can be used to smooth the ablation at a significant distance from the edge. More specifically, known techniques for ablating materials typically create small irregularities or unintended features in the interior of the ablation. The blurred edges will often be produced by imaging an aperture with an imaging system arranged so that the surface of the eye (or other target material) is beyond the imaging system""s depth of field. This can minimize the internal irregularities both at the microscopic level, as can be detected by optical interferometry, and at the macroscopic level, as can be detected by unaided human vision.
In a first aspect, the present invention provides a method for sculpting a material to effect a predetermined change in shape. The method comprises simultaneously directing a plurality of beams of ablative energy toward the material while the beams are directed at differing areas. As a result, the beams ablate a pattern from the material. The beams are manipulated so as to modify the ablation pattern. The ablative energy is then simultaneously directed along the manipulated beams.
In another aspect, the present invention provides a method for sculpting a material to effect a predetermined change in shape. The method comprises simultaneously directing ablative energy toward the material along a plurality of beams, each beam impacting at an associated ablation area of the material. The ablation areas only partially overlap, ideally so that a significant portion of the ablations extend beyond each other.
In another aspect, the invention includes a method for sculpting a material to effect a predetermined change in shape. The method comprises radiating a beam of ablative energy. A region of the material absorbs the ablative energy and ablates, the ablation having an edge. The ablation is shaped at a distance from the edge by blurring the edge.
The shaping of the ablation may include smoothing the ablation. Surprisingly, blurring the edge can smooth an internal portion of the ablation that is surrounded by, but separated from, the blurred edge. Preferably, a laser lases to create pulses of the ablative energy. The beam is formed from these pulses, and may be profiled with an aperture. An image of the aperture may be projected toward the material with an imaging system, and the edge may be blurred by positioning the material away from the depth of field of the imaging system. The blurring may be controlled by a collimating lens, and the imaging system may focus the beam to a waist near the back focal point of the imaging system.
In another aspect, the present invention provides a method for sculpting a lens in a region of a cornea of an eye. The method includes radiating a beam of an ablative energy by lasing a laser to produce a laser beam of the ablative energy. The beam comprises a multiplicity of laser beam pulses, and the energy is absorbed within a region of the cornea so as to ablate the region. The ablation has an edge, and an internal portion of the ablation can be smoothed by blurring the edge from a single pulse, the internal portion being surrounded by and separate from the blurred edge.
The present invention also provides a laser system for sculpting a region of a cornea to effect a predetermined change in shape. The system comprises at least one laser for generating a plurality of laser beams suitable for ablation of the cornea. An optical train is optically coupled to the beams so as to direct the plurality of optical beams toward differing areas of the cornea. The differing areas define an ablation pattern, and the optical train includes an adjustment mechanism for altering the ablation pattern.
In a further aspect, the invention provides a laser system for sculpting a region of a cornea to effect a predetermined change in shape. The system comprises a laser for generating a first beam of laser energy suitable for ablation of the cornea. An optical train is coupled to the first beam, the optical train including an optical element which separates the first beam into a plurality of laser beams. The optical train directs the plurality of optical beams toward partially overlapping areas of the cornea.
Therefore, it is an object of the invention to sculpt a material to effect a predetermined change in shape by partially overlapping a plurality of simultaneous laser beams. It is a further object of the invention to smooth an internal portion of an ablation that is not part of an edge of the ablation by blurring an edge of the ablation.